kbuild: remove kconfig binaries during make mrproper

Nigel Cunningham <nigel@nigel.suspend2.net> noticed
that 'make mrproper' did not remove mconf.
Fixed so we now remove all relevant binaries.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc:  Nigel Cunningham <nigel@nigel.suspend2.net>